This invention relates to roller chain drives for use in automotive engine chain drive systems that reduces the noise level normally associated with chain drives. More particularly, the invention is directed to a roller chain sprocket having an asymmetrical tooth profile that attempts to minimize the impact noise generated by the chain-sprocket collision during meshing.
While the invention is particularly directed to the art of roller chain sprockets for use in automotive engine camshaft drive applications, and thus will be described with specific reference thereto, it will be appreciated that the invention may have utility in other fields and applications.
Roller chain sprockets used in camshaft drives for automotive engines are typically manufactured to the International Standard ISO 606-1994(E). As shown in FIG. 1, the ISO 606 tooth profile is symmetrical with respect to the tooth space and has a constant root or roller seating radius R.sub.i extending from one tooth flank to the adjacent tooth flank as defined by the roller seating angle .alpha.. Accordingly, the flank radius R.sub.f is tangent to the roller seating radius R.sub.i at the tangency point TP. A chain with a link pitch P has rollers of diameter D.sub.1 in contact with a sprocket having a chordal pitch of P, a root diameter D.sub.2 and Z number of teeth. The pitch circle diameter PD, tip or outside diameter OD, and tooth angle A (equal to 360.degree./Z) further define the ISO-606 compliant sprocket.
FIG. 2 shows a typical cam-in-block ISO-606 compliant roller standard chain drive system 10, without a tensioner or chain guide, rotating in a clockwise direction as shown by arrow 11. The chain drive system 10 is comprised of a 25-tooth drive sprocket 20, a 50-tooth driven sprocket 30 and roller chain 40 having rollers 42. The roller chain 40 engages and wraps about sprockets 20 and 30 and has two spans extending between these sprockets, slack strand 44 and taut strand 46. The chain is under tension as shown by arrows 50, and distance D separates the centers of sprockets 20 and 30.
It is believed that a worn chain meshing with an ISO standard sprocket will have only one roller in driving contact and loaded at a maximum loading condition. This contact at maximum loading occurs as the roller enters the drive sprocket wrap 60 at engagement, shown in FIG. 2 as engaging roller 52. A second roller 54 is adjacent to the first roller 52 and is the next roller to mesh with the drive sprocket 20. The loading for roller 52 is composed primarily of the meshing impact loading and a major part of the chain tension 50 loading. The next several rollers in the wrap 60 forward of roller 52 share in the chain tension loading, but at a progressively decreasing rate. This loading of the roller 52 (and to a lesser extent for the next several rollers in the wrap) serves to maintain a solid or hard contact of the roller with the sprocket tooth surface. Roller 56, the last roller in the drive wrap 60 just prior to entering the slack strand 44, will also be in hard contact with drive sprocket 20, but at some point higher up on the root surface 22. With exception of rollers 52 and 56, and the several rollers following roller 52 that share the chain tension loading, the remaining rollers will not be in hard contact with the sprocket teeth and will be free to vibrate against the sprocket tooth surface as they travel around the wrap, contributing to the broadband mechanical noise level.
For the driven sprocket 30, the last roller in the wrap before it enters the taut strand, roller 59 in FIG. 2, is the roller in driving contact with the sprocket 30 at a maximum loading condition. And similar to the last roller in the drive sprocket wrap 60, roller 56, engaging roller 58 will be in hard contact with the root radius 32 of driven sprocket 30, but generally not at the root diameter.
It is known that a system such as that shown in FIG. 2 will gradually wear during ordinary use. This wear is comprised of sprocket tooth face wear and chain wear. Chain wear can be characterized as pitch elongation. Since a worn chain has an increased pitch length, a corresponding increase in the sprocket pitch diameter is a preferred interface for a worn chain.
Chain drive systems, including those using the ISO 606 compliant sprockets, have several components of undesirable noise. A major component of chain drive noise is the noise generated as the chain roller leaves the span and collides with the sprocket during meshing. At meshing, the roller will have a radial impact component as it collides with the sprocket tooth at its root, and a tangential impact component as it collides with the engaging tooth flank. It is appreciated that the loudness of the impact noise will be a function of the impact energy that must be absorbed during meshing. This impact energy is related to engine speed, chain mass, and the chain-sprocket engagement geometry, of which the engaging flank pressure angle is a major factor. The resultant meshing impact sound is repeated with a frequency generally equal to that of the frequency of the chain meshing with the sprocket.
Another source of chain drive noise is the broadband mechanical noise caused in part by shaft torsional vibrations and slight dimensional inaccuracies between the chain and the sprockets. Contributing to a greater extent to broadband noise level is the intermittent or vibrating contact between the unloaded rollers and the sprocket teeth as the rollers travel around the wrap.
It is known that providing tooth space clearance between sprocket teeth promotes hard contact between the chain rollers and sprocket in the sprocket wrap, even as the chain wears. This has the effect of reducing broadband mechanical noise component of the overall chain drive system noise. Adding tooth space clearance between sprocket teeth does not, however, reduce chain drive noise caused by the impact of roller-sprocket engagement.
Accordingly, there is a need for a chain drive system that will reduce the noise caused by the impact of the chain rollers during sprocket engagement. The present invention satisfies this and other needs associated with conventional chain drive systems. For example, the present invention will utilize an asymmetrical tooth profile with a decreased pressure angle for the engaging flank to reduce the roller-sprocket tangential impact force and the resultant noise level associated with the tangential roller impact. This asymmetrical tooth profile will also utilize a reduced roller seating angle at the disengaging flank to promote faster roller-sprocket separation as the roller exits the sprocket wrap. This reduced roller seating angle will also beneficially maintain the rollers in solid or hard contact with the sprocket tooth surface while the rollers are in the wrap, thereby contributing to a reduced broadband mechanical noise level. For the preferred embodiments of the present invention, an inclined root surface is used to incorporate tooth space clearance to beneficially reduce the roller-sprocket radial impact force and the resultant noise level associated with radial roller impact. The inclined root surface will also beneficially maintain the roller-sprocket hard contact even as the chain pitch elongates with wear, further contributing to a reduced broadband mechanical noise level. Other needs satisfied by the present invention will become apparent to the skilled artisan from the discussion of the preferred embodiments.